The present invention relates to an assembly of sheet materials in which a pair of sheets of materials is overlappingly joined, a tube assembly comprising tubular bodies which are overlappingly joined to one another, a drawing method for obtaining the sheet assembly and the tube assembly, and tools used in the drawing method.
An example of the related art is a suspension strut (a piston-cylinder assembly) such as that shown in FIG. 16.
A suspension strut comprises a piston 1, an inner cylinder 2 in which the piston 1 is slidably provided, an outer cylinder (a tube) 3 having one end closed, which accommodates the inner cylinder 2, and a piston rod 4 having one end connected to the piston 1. The other end of the piston rod 4 extends to the outside through a rod guide 5 commonly fitted into the open end portions of the inner cylinder 2 and the tube 3. A hydraulic fluid is sealably contained in the inner cylinder 2. The suspension strut further comprises damping force generating means (not shown) provided in the piston 1 and a base valve 6 attached to a lower end of the inner cylinder 2. The hydraulic fluid sealably contained in the inner cylinder 2 is flowed through the damping force generating means and the base valve 6, to thereby generate a damping force during extension and compression strokes. A gas and the hydraulic fluid are sealably contained in a reservoir 7 provided between the inner cylinder 2 and the tube 3, so as to compensate for a hydraulic fluid in an amount corresponding to the volume of the portion of the piston rod 4 which enters or exits the inner cylinder 2.
In a strut of this type (a dual-tube type hydraulic cylinder), a spring seat 11 is overlappingly joined to the tube 3 at an intermediate position between axially opposite ends of the tube 3, and a knuckle bracket 12 is overlappingly joined to a lower end of the tube 3. The spring seat 11 receives a spring provided between the spring seat 11 and a vehicle body. The knuckle bracket 12 is connected to a knuckle of the vehicle body. Generally, the spring seat 11 and the knuckle bracket 12 (support members) are joined by welding to the outer cylinder 3 (the tube), at an end of the overlapping part of the tube 3 and each of the support members 11 and 12 in a circumferential direction, as indicated by weld joints 8. In a joint of this type, however, the tube 3 is undesirably deformed due to heat produced during the welding process, resulting in poor dimensional accuracy. Further, a welding operation is time-consuming. In addition, oxide scales are formed on an inner surface of the tube 3 due to heat produced during the welding process, and particles of dust are spread into the air during welding. These oxide scales or particles of dust contaminate the hydraulic fluid. Thus, a weld joint is disadvantageous in terms of dimensional accuracy, productivity and durability.
Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 09-060682 discloses a joining method in which an overlapping part of a knuckle bracket (a first tubular body) and a tube (a second tubular body) is pressed radially inwardly at a plurality of positions under electric resistance heating, thereby forming joint portions as spot-like recesses. By this joining method, the above-mentioned disadvantages of a weld joint can be avoided.
As a drawing method for joining two sheets of materials, U.S. Pat. No. 4,831,704 discloses a method in which an outer wall portion of a drawn portion of one sheet located rearward relative to a direction of drawing is laterally extruded into a drawn portion of the other sheet located forward relative to the direction of drawing. However, there is no case in which this drawing method is applied to the joining of tubular bodies.
The above-mentioned prior art techniques are disadvantageous for the following reasons.
The joint portion of Kokai No. 09-060682 has a cup-like shape in which a convexly curved surface of the first tubular body (knuckle bracket) and a concavely curved surface of the second tubular body (outer cylinder) simply make contact with each other to form a cup-like joint. This results in poor peeling resistance. Therefore, when a large peeling force acts on the first tubular body, the first tubular body is readily peeled from the second tubular body.
In the drawing method of U.S. Pat. No. 4,831,704, a joint portion having a laterally extruded shape is obtained, enabling high peeling resistance. Therefore, if this drawing method is applied to the joining of tubular bodies, the above-mentioned problem of low peeling resistance is solved. However, the method of U.S. Pat. No. 4,831,704 involves reduction of a thickness of the sheet due to plastic flow of material. Therefore, if the method of U.S. Pat. No. 4,831,704 is simply applied to the joining of tubular bodies, a lowering of shear strength cannot be avoided. Therefore, the method of U.S. Pat. No. 4,831,704 cannot be applied to a member subject to a large shearing force, such as a strut.